Length
Convert between length units like light years (ly), miles (mi), kilometers (km), feet (ft), meters (m), and yards (yd) instantly. You can use the converters below, or search for a specific length conversion (e.g. kilometers to miles).
Metric system (SI) ↔ Imperial system
Metric system (SI)
Imperial system
Astronomy
Each converter is customizable. For instance, you have the option to switch between any available length unit, which makes it possible to convert between units, even if there isn’t a dedicated page for that combination yet (such as light years to inches).
Understanding length units
Length is a fundamental measurement used to describe distance, height, width, and depth. However, people worldwide use different measurement systems, while the metric system (SI) is the most common and widely accepted standard.
The metric system (SI)
The metric system, also called the International System of Units (SI), is used in most countries. Its units are based on powers of ten, which makes conversions simple and consistent.
- Base unit of length: meter (m)
- Common length units: kilometers (km), centimeters (cm), millimeters (mm)
- Prefixes: Metric prefixes such as kilo-, centi-, and milli- allow easy scaling from large to tiny measurements
Imperial and US customary system
Imperial and US customary units are primarily used in the United States, and to a lesser extent still in the United Kingdom.
- Common units: miles (mi), yards (yd), feet (ft), inches (in)
- Conversion system: Not based on powers of ten, so each conversion requires a specific factor
Length conversion formulas
You can use these formulas to convert between common length units, but it won’t be as quick as using our conversion tools. Each formula uses the precise conversion factor to avoid rounding errors.
| From | To | Formula |
|---|---|---|
| Kilometers (km) | Miles (mi) | mi = km ÷ 1.609344 |
| Miles (mi) | Kilometers (km) | km = mi × 1.609344 |
| Meters (m) | Yards (yd) | yd = m ÷ 0.9144 |
| Yards (yd) | Meters (m) | m = yd × 0.9144 |
| Meters (m) | Feet (ft) | ft = m ÷ 0.3048 |
| Feet (ft) | Meters (m) | m = ft × 0.3048 |
| Centimeters (cm) | Inches (in) | in = cm ÷ 2.54 |
| Inches (in) | Centimeters (cm) | cm = in × 2.54 |
FAQ
What are the common units of length?
Length is measured using units such as kilometers (km), meters (m), centimeters (cm), millimeters (mm), miles (mi), yards (yd), feet (ft), and inches (in). For example, centimeters and inches are often used for small measurements, while kilometers and miles cover longer distances.
Which unit is the base SI unit of length?
The meter (m) is the base SI unit of length. You can use SI prefixes like milli-, centi-, and kilo- to create smaller or larger units, such as millimeters, centimeters, and kilometers.
How do the metric system and the imperial system differ?
The metric system uses units based on powers of ten, making conversions straightforward. In contrast, the imperial system, relies on units which require specific conversion factors.
How is length used in everyday life and science?
People measure length to find height, room dimensions, distances between locations etc. Scientists and engineers also use precise length measurements in experiments, construction, design projects, and alike.
Why do different length measurement systems exist?
Different length systems developed historically in various regions. The metric system was later standardized internationally, while the imperial system persisted in countries like the United States for cultural and historical reasons.
What are some unusual or non-standard length units?
Unusual length units include for instance parsecs, light years, astronomical units, and nautical miles. These units are mainly used in astronomy, navigation, and specialized scientific contexts, rather than for everyday measurements.